Charged particle beam devices are becoming increasingly important for imaging and structuring micro- and nanometer sized structures and devices. While electron beams are preferred for imaging, ion beams are more suitable for machining a specimen, for example, by using the ion beam for etching, cutting or deposition.
For inspecting or structuring a specimen efficiently with a high spatial resolution, it is important that the aperture of the charged particle beam device is well matched to the operational set up. For example, for obtaining minimum charged particle beam spot size, or a maximum beam current at a given beam spot size, the aperture has to be optimized with respect to aberrations of the lenses involved in the charged particle beam device, to diffraction which depends on the wavelength of the charged particles, to particle beam current which influences Coulomb interaction, and to system magnification.
FIG. 1 illustrates schematically, as an example, a scanning charged particle beam device 1 having a charged particle beam source 5 that emits a charged particle beam 7, an extraction electrode 11 to accelerate the charged particles of the charged particle beam 7 to a desired beam energy, an aperture system 13 to define aperture angle α and beam current, and a focussing lens 9 to focus the charged particle beam onto a specimen 3. For completeness, FIG. 1 also depicts a scanning system 17 to scan the charged particle beam 7 across the surface of the specimen 3.
The aperture system 13 of FIG. 1 depicts schematically three different circular apertures 13a, 13b and 13c which enable a person to operate the charged particle beam device at three different beam currents and aperture angles α. In the case of FIG. 1, the aperture angle α is defined by the maximum angle with respect to the optical axis 8 at which a ray of charged particles can pass through the opening 13a. Accordingly, the aperture angle α is defined by the diameter D of the opening 13a, and the distance L between the charged particle beam source 5 and the opening 13a The three different apertures 13a, 13b, 13c can be selected by using aperture drive 15 to linearly move one of the three apertures into the charged particle beam 7.
The openings of the apertures 13a, 13b, 13c, of FIG. 1 are circular to provide that the respective aperture can be aligned to be fully rotational symmetric with respect to the optical axis 8. With full rotational symmetry, the aperture angle α is independent of the plane within which the aperture angle α is taken. Therefore, a fully rotationally symmetric aperture usually provides the highest focussing quality compared to systems with apertures of less rotational symmetry.
However, the aperture system 13 of FIG. 1 with the three opening 13a, 13b, 13c, allows for only three different aperture angles α to optimize beam current and beam resolution. While it is true that aperture system 13 may be designed to have more than the three apertures of FIG. 1, the total number of apertures of a aperture system is always limited by tight space limitations and the constraint not to deter the electric field configuration within the beam column.
Further, when shifting aperture system 13 to change from one aperture with a first diameter D to another aperture with a second diameter, beam operation is interrupted. Such interruptions make it difficult to adjust the aperture during operation. In addition, changing the aperture by shifting aperture system 13 requires each time an alignment procedure to align the new aperture to the optical beam axis. Such alignment procedure is generally time consuming.
Further, permanent exposure of the aperture system 13 of FIG. 1 to a charged particle beam usually causes the aperture defining edges to change over time. For example, exposure to an electron beam generally leads to a contamination of the edges, while exposure to an ion beam generally leads to a removal of the aperture defining material. Both effects cause the aperture angle to drift over time which in turn causes beam spot size and beam current to vary uncontrollably.